Actually, the "antlers" are part of the wiref ramework which shapes the leaflets. The CE pericardial valve is does not come from the cow heart itself, but rather the pericardial sac surrounding the heart. They cut individual leaflets from the tissue and the shape of the wires, rubber, and dacron used in the valve cause the tissue to form into leaflets of the valve.
From my understanding, the "stent" is the supporting part or the ring, designed to give porcine tissue valves addtional support at the sewing area of the valve.